Update docs to mention Worker Key

[tir38]

We talk about Worker identity in three places:

https://square.github.io/workflow/userguide/core-worker/#workers-define-their-own-equivalence

For these reasons, any time a workflow requests that a worker be run in sequential render passes, it is asked to compare itself with its last instance and determine if they are equivalent.

https://square.github.io/workflow/swift/api/WorkflowReactiveSwift/protocols/Worker/

https://square.github.io/workflow/kotlin/api/gfmCollector/workflow/com.squareup.workflow1/-worker/does-same-work-as/

Some or all of these documentations should also mention "Key" as a unique identifier when calling context.runningWorker

https://square.github.io/workflow/kotlin/api/gfmCollector/workflow/com.squareup.workflow1/running-worker/